I call those hours "PR HR" - "Public Relations hours".

It is the cost of doing business.

Those 50 hours may have made you $100,000 is sales because the word of mouth it generated from one client to the other. It can be hard to put a value on it.

I avoid any service contracts, like mentioned above, if people are paying for a service contract no matter what is written in it they will demand service immediately and if you can't get there within "their" timeline, you have an angry client.

When bidding a job, I can get a feel for a client's/system's neediness and will add a couple of hours onto the job cost for phone calls and the occasional onsite visit.

Basically a service contract without the obligation.